Polymers capable of reversibly complexing acid gases and a method of using the same

ABSTRACT

Aminated polymers capable of reversibly complexing carbon dioxide and other acid gases including CO 2 , SO 2 , NO x  and H 2  S, a method of using such polymers and an operational unit including such polymers are disclosed. The polymers incorporate amine group which reversibly complex acid gases. Such polymers provide a significant advantages over presently used sorbents used to remove acid gas.

This is a continuation-in-part of copending application Ser. No.08/028,373, filed on Mar. 9, 1993.

FIELD OF THE INVENTION

The present invention is related to polymers capable of reversiblycomplexing acid gases and to a method of using such polymers as acid gassorbents, and especially to environmentally preferred, aminated polymersthat are capable of reversibly complexing acid gases including CO₂, SO₂,NO_(x) and H₂ S and to a method and operational unit for using suchpolymers.

BACKGROUND OF THE INVENTION

A significant portion of the domestic foamed polystyrene industry isthreatened by environmental problems associated with production and useof expanded, or foamed polystyrene. Foamed (also known as "expanded" or"cellular") polystyrene is produced via physical blending of "blowingagents with the polymer matrix, followed by heat treatment to initiatefoaming. Blowing agents can be sub-divided in to two general classes:(1) small molecules which thermally decompose to a gas (predominantlynitrogen) which initiates foaming, plus other volatile fragments; and(2) volatile liquids which are absorbed into the polymer, and whichsubsequently foam the polymer upon heating simply via vaporization.Examples of the first class of blowing agents include azo compounds(nitrogen producing) and calcium carbonate (CO₂ -producing). Examples ofthe second type of blowing agent include Freons, pentane, air, nitrogen,and carbon dioxide.

The cells formed in the polymer during foaming remain filled with vaporcharacteristic of the blowing agent used. Given the large volume of foamproduced each year, diffusion of vapor into the atmosphere during thefoamed product's lifetime can cause significant environmental problems.Further, refoaming of recycled foamed polystyrene entails production anduse of additional blowing agent which is both expensive and increasinglyenvironmentally unacceptable.

Conventional foamed thermoplastics are produced via two distinctprocesses. In the first process, blowing agent is added either justprior to, or during extrusion of the polymer. High pressure within theextruder maintains a homogeneous mixture of blowing agent gas andpolymer. Foaming commences with the reduction of pressure upon thepolymer exiting the extruder. In the second process, blowing agent isadded to polymer beads, which are then stored and shipped to a molder.Upon heating in a mold, the beads expand because of the action of theblowing agent, producing a molded, foamed article such as the drinkingcups and trays used in many food-service applications.

Until recently, the preferred blowing agent for use in polystyrene beadswas a Freon (chlorofluorocarbon) material or mixture. Freons werepreferred because of their low toxicities, low flammabilities, and lowboiling points. However, the publicity surrounding the effect of Freonson the atmospheric ozone layer, followed swiftly by the Montrealprotocols, prompted the plastic industry to replace Freons in expandedpolystyrene by a volatile alkane, usually pentane. Because pentane isnot a "natural" material (it is refined from petroleum) and is also a"greenhouse gas", concern has arisen over the climactic effects ofsignificant amounts of pentane released into the atmosphere as a resultof foamed polystyrene production. The plastics industry has thus beensearching for environmentally acceptable replacements for pentane infoamed polystyrene.

From an environmental perspective, CO₂ is unquestionably an attractivethermoplastic blowing agent in that it can be readily recovered from theatmosphere, it is non-flammable, and it exhibits relatively lowtoxicity. Indeed, experience has shown that carbon dioxide can produce acellular morphology when used as a blowing agent for polystyrene. U.S.Pat. No. 4,925,606, German Patent No. 3,829,630, U.S. Pat. No.4,911,869, Japanese Patent No. 63,000,330, Zwolinski, L. M., Dwyer, F.J., 42 Plast. Eng. 45 (1986) and French Patent No. 2,563,836 discuss theuse of carbon dioxide as a blowing agent in extruded polystyrene foam.

As indicated by Wissinger, R. G. and Paulairis, M. E., 25 J. Polym.Sci.: Part B: Polym. Phys. 2497 (1987) polystyrene will indeed absorbsignificant amounts of CO₂ under pressure. Because of the relatively lowsolubility of CO₂ in polystyrene and its high volatility, however, mostof the gas rapidly effuses from the polymer matrix upon reduction of thepressure. This rapid effusion prevents formation of a commerciallyacceptable foam. Commercial producers of extruded polystyrene foam,therefore, use either Freon, pentane or a mixture of Freon or pentaneand CO₂ as a blowing agent.

Moreover, despite the limited success of CO₂ in extruded foam, eitheralone or in a mixture, the use of CO₂ in foamed polystyrene beads is notpresently possible. Quite clearly, unlike pentane and freon, theequilibrium concentration of CO₂ in polystyrene at atmospheric pressureis too low to support subsequent foaming of polystyrene beads orsecondary foaming of slabstock during thermoforming.

In an art unrelated to foamable polymers, it is known that low molecularweight primary and secondary amines will react with CO₂ to form carbamic"zwitterions," providing the amines are sufficiently basic in character.These reactions have been discussed by Javier, F.J.B.G., Ing. Quim. 317(Oct. 1989); Javier, F.J.B.G., Ing. Quim., 215 (Nov. 1989); Danckwerts,P. V., Sharms, M. M., 10 Chem. Eng. 244 (1966); Laddha, S. S.,Dankwefts, P. V. 37 Chem. Eng. Sci. 475 (1982); Versteeg, G. F., anSwaaij, W.P.M., 43 Chem. Eng. Sci. 573 (1988) and Danckwerts, P. V., 34Chem Eng. Sci 443 (1979). It is also known that these zwitterions arestable at ambient conditions but will revert to carbon dioxide and freeamine at higher temperatures. U.S. Pat. No. 3,029,227; U.S. Pat. No.3,423,345; U.S. Pat. No. 4,102,801. CO₂ /amine reactions have previouslybeen used to construct thermally reversible protecting groups forreactive epoxy systems and to selectively remove carbon dioxide and acidgases from gas streams.

General public awareness concerning the protection of the environmenthas created the need to devise environmentally friendly andenergy-efficient technology for the clean-up of industrial gas streams.Weakly acidic gases such as CO₂, SO₂, NO_(x) and H₂ S dischargeddirectly into the atmosphere have been suggested to contribute to theformation of acid rain and so-called greenhouse warming. In addition,acidic gases such as CO₂, SO₂ etc. can act as poisons for variouscatalyst systems and thus must be removed from certain process streams.Traditional methods for acid gas removal include:

1. Aqueous solutions of amines/alkanolamines for scrubbing CO₂ asdiscussed above. The main disadvantages of these sorbent systems are:Slow reaction rates, energy intensive regeneration step (must heat largevolumes of water), side reactions which degrade the amines, and loss ofamines by evaporation.

2. Limestone for the removal of SO₂ ; the major environmental drawbackassociated with this process is the generation of large quantities ofsludge.

3. High temperature mineral sorbents for the removal of SO₂ and NO.These materials are however not designed to also remove CO₂ selectively.

It is an object of this invention to provide environmentally safe,foamed polymers and especially to provide a polymeric matrixincorporating pendant amine groups capable of reversibly complexing CO₂,thereby providing an environmentally safe method of producing anexpanded or foamed polymer.

It is also an object of this invention to provide polymeric matricesincorporating amine groups useful generally as thermally reversiblesorbents for acid gases.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides polymers capable ofreversibly complexing carbon dioxide and other acid gases including SO₂,H₂ S and NO_(x).

In a preferred embodiment, an expandable (foamable) thermoplasticmaterial is provided that is inherently recyclable and emits no pentane,freon or other environmentally hazardous gas to the atmosphere, eitherduring or after the foaming process. The present invention also providesa method of producing such a thermoplastic material.

It has been discovered that the ability of amines to complex or reactwith carbon dioxide is substantially unaffected by incorporation of suchamines into a polymeric matrix. Indeed, the thermal stability ofamine-CO₂ adducts appears to increase upon incorporation of the amineinto the polymer matrix. For example, tertiary amine-functional polymersbind CO₂ whereas a low molecular weight tertiary amine does not.

Generally, the aminated or amine-functional polymers react with CO₂ toform carbamic zwitterions (equation 1 below) and/or ion/counterion pairs(equation 2 below) of the following general formulae: ##STR1##

Wherein R is a polymer preferably having a weight average molecularweight greater than approximately 25,000. Preferably R' also comprise anamino group capable of complexing with CO₂ to form a carbimic zwitterionand/or an ion/counterion pair.

Aminated polymers with softening or glass transition temperatures (T_(g)'s) in a range coinciding with the range of temperatures over which CO₂reaction products thermally dissociate will form a foamed polymer viathermal dissociation of the CO₂. Upon heating the material above apredetermined temperature, the polymer softens and the zwitterionsand/or ion/counterion pairs revert to "free" amine and CO₂, therebyinducing foaming.

Thus, polymers containing amine groups (preferably pendant primary orsecondary amine groups) are first synthesized. Any amine group capableof reversibly complexing carbon dioxide can be used. Preferably theamine groups have a pK_(a) greater than approximately 8.0. The pendantamine groups are preferably di- or triamines. Suitable amine groupsinclude but are not limited to piperazine (PIP), 1-methylpiperazine(1-MPIP), cis and trans 1,4-diaminocyclohexane (1,4-DAC),diethylenetriamine (DETA), TETA, hexamethylene diamine, ethylenediamine(EDA), N-MEDA, N,N-DMEDA, N,N'-DMEDA, N,N,N'-TMEDA, diamine-p-menthaneadamantanediamine, N,N'-diethyl-2-butene-1,4-diamine,N-cyclohexyl-1,3-propanediamine, and 3,3'-diamino-N-methyldipropylamine.Most preferably, the pendant amine groups are diamines.

Any polymer comprising pendant amine groups which can be thermallysoftened and foamed is suitable in that CO₂ binding or complexing isaccomplished by the pendant amine groups. The polymer backbone functionsto anchor the CO₂ complexing pendant amine groups and does not hinderthe complexing or binding and subsequent debonding of CO₂.

To provide polymers with appropriate physical properties and T_(g) 'sfor foaming, the polymers preferably have a weight average molecularweight (M_(w)) above approximately 25,000. Preferably M_(w) is in therange of approximately 25,000 to 1,000,000. More preferably, thepolymers have a weight average molecular weight in the range ofapproximately 75,000 to 1,000,000. Most preferably M_(w) is in the rangeof approximately 100,000 to 1,000,000.

The present aminated polymers preferably comprise sufficient aminegroups to complex sufficient carbon dioxide to effect a desired degreeof foaming. Preferably, the polymers comprise sufficient amine group tocomplex between approximately 1-33 weight percent carbon dioxide basedupon the weight of the uncomplexed aminated polymer. More preferably thepolymers comprise sufficient amine groups to complex betweenapproximately 4-20 weight percent carbon dioxide. Most preferably thepolymers comprise sufficient amine groups to complex betweenapproximately 5-20 weight percent carbon dioxide.

Preferably, a thermoplastic material is produced by the copolymerizationof vinyl monomers which can react with carbon dioxide to form carbamiczwitterions and/or ion/counterion pairs. More specifically, monomerswhich can be copolymerized with styrene, or for that matter, any vinylmonomer, and which can reversibly complex carbon dioxide aresynthesized. Such vinyl polymers are relatively easily synthesized inthe laboratory. Moreover, unaminated vinyl polymers such as polystyrenepresently account for a large percentage of the foamed polymer market.

While stable under ambient conditions, the CO₂ reaction products of thepolymers thermally disassociate to form free CO₂ at temperatures aboveapproximately 60°-70° C.

Aminated polymers and copolymers can be exposed to CO₂ either in bulk orin solution. Aminated polymers are found to react readily with CO₂ undera variety of conditions including absorption of CO₂ from the air atambient conditions. The exposure to CO₂ results first in swelling,followed quickly by reaction and significant complexing of the CO₂ intothe material, preferably in the form of zwitterions. Unlike the case ofsimple swelling of polystyrene by CO₂, the stability of thesezwitterions at atmospheric pressure and temperature preventsdestabilization of the material upon a reduction in pressure to ambientconditions.

Thermal dissociation of the adsorbed CO₂ occurs in a clean fashion,resulting in the regeneration of the original aminated copolymer. Thebinding capacity of the pendant amine groups increases in the orderprimary > secondary > tertiary following increases in relative basicity.

Because the reaction of an amine and CO₂ to produce a carbamiczwitterion and/or an ion/counterion pair is a reversible reaction, andbecause the amines are locked into the polymer backbone, the presentpolymers can be reverted to their expandable form simply by collection,washing, granulating, and re-exposure to high pressure CO₂.

Further, because the carbon dioxide can be isolated from the atmosphere,the product will be recycling the gas continually, rather thangenerating new gases. Reversion to the expanded polymer closes therecycling loop for this material, a major goal of plastics recyclers.

Still further the ability in the present invention to accurately varythe amine content of the aminated polymers and thereby the amount of CO₂complexed enables substantial control over foaming and thereby over thephysical properties of the foamed product.

Moreover, microporous as well as linear, non-porous amine-functionalpolymers and copolymers are effective, thermally-reversible sorbents foracid gases including CO₂, NO_(x), SO₂ and H₂ S. Regenerable polymericsorbents for acidic gases provide several advantages in industrialapplications because they are environmentally benign, relatively stable,easy to handle (low density), recyclable, and can be regenerated undermild reaction conditions. In addition, the ability to prepare highlyporous, crosslinked materials maximizes both the rate and ultimatecapacity for binding, significantly enhancing the potential of thesematerials for industrial applications.

The reaction products resulting from the reaction of carbon dioxide withan aminated polymer have been set forth above. The reaction product ofsulfur dioxide and an aminated polymer has the following general formulawhich can be referred to as a sulfamic zwitterion: ##STR2## The reactionproduct of nitric oxide and an aminated polymer is believed to have thefollowing general formula which can be referred to as a nonoate:##STR3## The reaction product of hydrogen sulfide and an aminatedpolymer is believed to have the following general formula which can bereferred to as an ion/counterion pair: ##STR4##

In aminated polymeric each of the above equations R is a polymer. R maybe a linear aminated polymer as described above in connection with thereactive complexing of CO₂. Preferably, R is an aminated crosslinked,microporous polymer. Once again, the amine groups of the aminatedpolymer are preferably pendant amine groups. As the polymer backboneacts essentially to anchor the pendant amine groups and does not hinderthe binding and subsequent debonding of the acid gas, substantially anypolymer backbone is suitable. Generally, the preferred choices of aminegroups are as described above in connection with the reactive complexingof CO₂ to form foamed polymers.

These materials may also find potential applications in facilitatedtransport membrane technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a Scheme 1 polymer synthesis under thepresent invention.

FIG. 2 is an illustration of a Scheme 2 polymer synthesis under thepresent invention.

FIG. 3 is an illustration of two scheme 3 polymer syntheses under thepresent invention.

FIG. 4 is an illustration of a Scheme 4 polymer synthesis.

FIG. 5 is an illustration of a copolymer of styrene andpiperazine-functionalized styrene.

FIG. 6 is an illustration of a copolymer of styrene and hexamethylenediamine.

FIG. 7 is a ¹³ C NMR spectrum for a copolymer functionalized with1,4-DAC.

FIG. 8A is an illustration of a ¹³ C-NMR spectrum of an S-VBC copolymer.

FIG. 8B is an illustration of a ¹³ C-NMR spectrum of an EDA-functionalcopolymer.

FIG. 9A is an illustration of an ¹ H-NMR spectrum of an S-VBCcopolymers.

FIG. 9B is an illustration of a ¹ H-NMR spectrum of an EDA-functionalcopolymer.

FIG. 10 is an illustration of the glass transition behavior ofstyrene/vinylbenzylchloride (S-VBC) copolymers as a function of VBCcontent.

FIG. 11 is an illustration of the glass transition behavior ofEDA-functional copolymers.

FIG. 12A is an illustration of a DSC scan of CO₂ release byEDA-copolymer/CO₂ reaction products.

FIG. 12B is an illustration of a TGA scan of CO₂ release byEDA-copolymer/CO₂ reaction products.

FIG. 13 is a graphical illustration of the CO₂ capacity ofEDA-functional copolymer as a function of EDA content.

FIG. 14 is a graphical illustration of desorption onset temperature(T_(d)) as a function of pressure.

FIG. 15 is an illustration of FTIR spectra of EDA-functional copolymer;the upper curve is for film following CO₂ exposure and subsequent heattreatment; the lower curve is for film after CO₂ exposure alone.

FIG. 16 is an illustration of mass spectroscopy data collected duringthermally-initiated release of CO₂ from EDA-copolymer/CO₂ reactionproduct.

FIG. 17 is an illustration of a synthesis scheme for crosslinked beads.

FIG. 18 is an illustration of FTIR spectra of precursor versus aminatedproduct.

FIG. 19 is an illustration of calibration curve of a mass spectrometer.

FIG. 20 is an illustration of binding/debonding of CO₂, monitored bymass spectroscopy.

FIGS. 21A-21D are illustrations of binding/debonding of SO₂ and NOrecorded by TGA.

FIG. 22 is an illustration CO₂ binding exotherm recorded by DSC.

FIG. 23 is an illustration of CO₂ debonding endotherm.

FIG. 24 is an illustration of a CO₂ binding capacity as a function oftemperature.

FIG. 25 is an illustration of fraction conversion of primary aminegroups reacted versus amine content.

FIG. 26 is an illustration of CO₂ binding capacity versus amine content.

FIG. 27 is an illustration of versus d/R_(p) amine content.

FIG. 28A is an illustration of a typical TGA binding isotherm.

FIG. 28B is a detailed illustration of Region I of a typical TGA bindingisotherm.

FIG. 29 is an illustration of (FWI-1.5*FWI^(2/3)) versus t.

FIG. 30 is an illustration of CO₂ transmittance (C*D) through productlayer versus amine content.

FIG. 31 is a pseudo-first order kinetic model plot of TGA data.

FIG. 32 is an illustration of fraction conversion versus t.

FIG. 33 is an illustration of rate of CO₂ binding versus CO₂concentration in the purge.

FIG. 34A is an illustration of an operational unit for acid gas removalincluding a sorbent system in the form of fixed bed.

FIG. 34B is an illustration of an operational unit for acid gas removalincluding a sorbent system in the form of fluidized bed.

FIG. 35 is an illustration of an aminated polymer coded substrate foruse in acid gas removal.

DETAILED DESCRIPTION OF THE INVENTION A. Complexing of CO₂ Using LinearAminated Polymers and Production of Foamed Polymers.

Results from studies of CO₂ /amine reactions using free amine indicate:(1) the reactivity of amines towards CO₂ appears to increase as thebasicity of the amine increases (pK_(b) decreases) and (2) varying thestoichiometry of the reaction can lead to structurally differentproducts. Regarding the latter point, a 2:1 amine/CO₂ ratio is generallythought to produce the ion/counterion product: ##STR5## whereas anequimolar ratio is predicted to generate the zwitterion: ##STR6## It ispreferable to complex at least one CO₂ molecule per amine group in thepresent polymers, thereby maximizing the number of CO₂ moleculesavailable for foaming. It is, therefore, preferable to form thezwitterion.

1. Monomer and Copolymer Synthetic Schemes

Amino-bearing functional groups can be incorporated into a polymerthrough several synthetic schemes. As depicted in Scheme I of FIG. 1,such functional groups can be incorporated via a copolymerization of avinyl monomer (e.g., styrene) and a functionalized vinyl comonomer(e.g., functionalized styrene). The functionalized comonomer(s) arereadily prepared either by addition of a primary or secondary amine tovinylbenzylchloride (VBC or chloromethyl styrene) (preferably in thepresence of a phase transfer catalyst) or by addition of anamine/lithiated amine mixture to divinylbenzene. A drawback to thepreparation of functionalized comonomer(s) using divinylbenzene is thedifficulty in obtaining sufficiently pure divinylbenzene. Diamines andtriamines are preferably used to ensure that an unreacted, pendant aminegroup remains to complex with CO₂. Monoamines have previously reactedwith cloromethyl styrene via a nucleophelic substitution of chlorine byN'Guyen, T. D., et al., 19 Polymer 423 (1978) and Tsuruta, T., et al.,177 Chem. 3255 (1976).

Amino-bearing functional groups may also be incorporated byfunctionalization of a previously prepared S-VBC copolymer (Scheme II asdepicted in FIG. 2). N'Guyen and coworkers have shown that monoaminescan be incorporated into a S-VBC copolymer. The latter procedure wasclaimed to ultimately produce a more thermally-stable polymer than thatderived from chloromethyl styrene. It has been discovered thatamino-functionalized styrene polymers can be readily prepared via thereaction of a poly styrene-co-vinylbenzylchloride or S-VBC with anexcess or di- or tri-amine in solution.

In a typical synthetic procedure under Scheme II, styrene and VBCmonomers supplied by Aldrich Chemical Company, Inc. are preferablywashed with dilute NaOH to remove inhibitor, then vacuum-distilled inthe presence of a drying agent and stabilizer. The S-VBC copolymer maybe prepared via solution polymerization in toluene under N₂. Thereaction is allowed to proceed for predetermined period of time, afterwhich time the copolymer is precipitated in a large excess of methanol,washed, then vacuum dried. Amino-functionalized styrene copolymers maybe prepared by adding a solution of poly (styrene-co-VBC or S-VBC) inDMF preferably to an excess of diamine or triamine. Excess diamineand/or triamine is preferably used to minimize crosslinking. Thereaction is allowed to proceed to completion at room temperature. .Theproduct is precipitated in dilute aqueous NaOH, washed, then vacuumdried.

Two additional synthetic schemes, referred to generally as Scheme III,for producing amino-functionalized copolymers are shown in FIGS. 3A and3B. In FIGS. 3A and 3B, a functionalized comonomer is prepared by thereaction of 1-(-isocyanoto-1-methylethyl)-3-(1-methylethenyl)benzene(TMI™) provided by American Cyanamid Company with a primary or secondaryamine or with water. The functionalized comohomer may then becopolymerized with styrene or another vinyl monomer.

Also consistent with Scheme III, a copolymer of TMI and styrene oranother vinyl monomer can first be synthesized. The resultant copolymercan then be functionalized in a reaction similar to those shown in FIGS.3A and 3B. In the case of copolymerization of styrene and TMI, thestyrene monomer is first washed to remove the inhibitor. TMI is obtainedinhibitor-free. The TMI and styrene are copolymerized in toluene atapproximately 70° C. under nitrogen using a free radical initiator suchas azo bis(isobutyronitrile) (AIBN). After approximately 36 hours, thecopolymer is withdrawn for analysis and functionalization. Thecopolymer/toluene solution is poured into a toluene solution containingan excess of di- or triamine. The mixture is then allowed to react atapproximately 50° C. overnight. Subsequently, the polymer isprecipitated by adding a large volume of the non-solvent methanol.Following precipitation, the copolymer is washed with methanol andacetone, and then vacuum dried.

In Scheme IV, depicted in FIG. 4, maleic anhydride is copolymerized witha vinyl monomer in the presence of a free radical initiator such as(AIBN). The copolymer is then functionalized by the addition of aprimary or secondary amine.

Finally, it has been demonstrated that certain amino acids, particularlylysine and arginine, effectively complex carbon dioxide. These resultssuggest the possibility of generation of amino-acid functionalizedsynthetic polymers which reversibly complex CO₂, or the intriguingpotential for the design of proteins or polysaccharides which can beused as recyclable and/or degradable, expandable materials.

2. Examples of Copolymer Synthesis Example A

In a synthetic procedure under Scheme I, an amine functionalized vinylcomonomer was produced by reaction of vinylbenzylchloride (VBC) withethylenediamine (EDA) in the presence of PolyDMAP™, a polymer-bounddialkylaminopyridine acylation catalyst (Reilly Industries, Inc.). Thissynthesis consisted of adding dropwise a solution of 10.7 g (70.2 mmols)VBC in 20 ml of dry toluene into a stirred mixture of 21 g (350 mmols)EDA, 5 ml of toluene and 500 mg of polyDMAP. After 30 minutes, thesolvent and excess EDA were removed under vacuum. The aminated comonomerwas separated from the catalyst by filtration; the yield wasapproximately 62%. The aminated copolymer was subsequently obtained viaa solution free-radical copolymerization. In a typical reaction, 13.6 g(131 mmols) of styrene, 4.9 g (28 mmols) comonomer and 18 mg of AIBNwere added to 40 ml of toluene under nitrogen at room temperature. Themixture was stirred at 68° C. for six hours after which the contentswere poured into a large volume (300 ml) of methanol to coagulate thepolymer. After subsequent filtration and washing with methanol, theproduct was dried for 24 hours under vacuum.

Example B

In a synthesis under the procedure of Scheme II, S-VBC copolymers wereprepared via AIBN-initiated solution copolymerization in toluene at 68°C. Several amines, including piperazine (PIP), 1-methylpiperazine(1-MPIP), cis and trans 1,4-diaminocyclohexane (1,4-DAC),diethylenetriamine (DETA) and ethylenediamine (EDA) have been reactedwith styrene-VBC copolymer (1 g, VBC/Styrene=1:5) in toluene (80 ml).The copolymer solution is dripped slowly into a large excess of amine in100 ml of toluene. In the presence of an acylation catalyst, the mixturewas stirred under nitrogen for several days at 30° C., after which thesolvent is removed under vacuum. The product was washed to remove excessamine and dried under vacuum.

Example C

In another synthesis under Scheme II, a copolymer of styrene andpiperazine-functionalized styrene shown in FIG. 5 (monomer/comonomermolar ratio=5:1) was successfully synthesized. This copolymer exhibiteda glass transition temperature of approximately 120° C. by DSC, whileTGA showed no weight loss up to 300° C. As in the case of the model freeamine compounds, the reactivity of the copolymer towards CO₂ was gaugedby dissolving a sample in chloroform and sparging the solution with CO₂.Exposure of the copolymer to CO₂ quickly led to formation of achloroform-insoluble product. A TGA analysis of this CO₂ reactionproduct showed weight loss commencing at leveled out at 8% at approx.140° C. (scan rate=20° C./min.). Examination of the TGA samplingfollowing cooling clearly showed a cellular structure. Curiously, aweight loss of 8% suggests that this copolymer complexes more than oneCO₂ molecule per amine group, given that the copolymer showed no signsof degradation following the TGA scan.

Example D

In another synthesis under Scheme II, a copolymer of styrene andhexamethylene diamine shown in FIG. 6 (monomer/comonomer molarratio=4:1) has been successfully synthesized. This copolymer exhibited aglass transition temperature of approximately 105° C. by DSC. Thepolymer was shown to be reversible to its carbon dioxide complexed formby re-exposure to high pressure CO₂.

Example E

In still another synthesis under scheme II, several copolymersfunctionalized with EDA variants were synthesized. Styrene and VBC (70%meta, 30% para) from Aldrich were washed separately with a 0.5% aqueoussodium hydroxide solution to remove the polymerization inhibitors. Thisprocess was repeated a minimum of three times. The monomers were thenrinsed with distilled water at least six times to substantiallyeliminate all traces of the sodium hydroxide. Molecular sieves wereadded to the monomers which were then stored for a minimum of 24 hoursin a refrigerator at 4° C. The monomers were then distilled under vacuumprior to polymerization.

The precursor copolymers were made in a toluene (Fisher) solution viafree radical polymerization initiated by azo-bisisobutyronitrile (AIBN)(Aldrich). Typically, a 3000 ml, three-neck round bottom flask mountedwith a reflux condenser was flushed with nitrogen for 1 hr. The reactorwas then charged with 100 ml toluene; 76.6 g (736.5 mmol) styrene; 22.5g (147.5 mmol) VBC and a solution of 105 mg of AIBN in 10 ml of toluene.The mixture was stirred at room temperature for 30 min while bubblingnitrogen into the reactants. The nitrogen feed was then shut off and thereactor was immersed in an oil bath preheated to 105° C. and thecontents stirred overnight. The copolymer was recovered by pouring thesolution into a large volume (100 ml) of methanol (Fisher). Theprecipitate was separated by filtration, redissolved in chloroform(Fisher) and coagulated again in methanol. The product was finallywashed several times with small volumes (50 ml) of methanol and driedunder vacuum.

The alkylation reactions to produce aminated copolymers were carried outin a toluene solution at 30° C. under nitrogen atmosphere in thepresence of polyDMAP. In a typical experiment, 5.259 g of an S-VBCcopolymer (with a 30% VBC molar content) were dissolved in 150 ml oftoluene. Meanwhile, 8.92 g (149 mmol) of EDA (Aldrich) (i.e., a largeexcess of EDA to VBC) and 20 ml of tolluene were placed in a 3000 ml,three neck round bottom flask previously flushed with nitrogen asindicated in the preceding paragraph. The copolymer solution was addeddropwise into the amine solution while stirring vigorously. The mixturewas stirred for 3 days, heated to 90° C. for 4 hrs. then poured into1500 ml of a 5% aqueous sodium hydroxide solution and stirred at roomtemperature overnight. To recover the copolymer, the solvent wasevaporated under vacuum and the water insoluble fraction of the aminocopolymer was separated by filtration, washed thoroughly with distilledwater then dried under vacuum. Preparation of materials functionalizedwith other ethylenediamine variants was also accomplished followingthese procedures.

3. Copolymer Characterization a. Composition

All polymers were characterized using infrared (IR) and ¹³ C NMR.Infra-red spectra were obtained on a Matson FT-IR as KBr pellets.Elemental analysis was performed by Galbraith Labs, Knoxville, Tenn.

A sample ¹³ C NMR spectrum obtained on a BRUCKER 300 MSL instrumentusing a 10 mm liquid probe and deuterochloroform as solvent is shown inFIG. 7 for the copolymer functionalized with 1,4-DAC as described inExample B. As can be seen, there are apparently no residual unreactedchloromethyl groups, consistent with elemental analysis results.Concerning the Scheme II copolymers of Example B, elemental analysisshows that the VBC/styrene ratio in the precursor copolymer is 1:5. GelPermeation Chromotog raphy () ) shows that the number average molecularweight (M_(n)) is approximately 75,000 and M_(w) is approximately128,000. Furthermore, elemental analysis of the aminated copolymersshows that nearly complete substitution the of chloromethyl group hasbeen achieved.

The compositions of the copolymers of Example E were also determined byelemental analysis at Galbraith Laboratories; Knoxville, Tenn., and alsovia high resolution proton-NMR (Brucker MSL300 instrument with a 5 mmhigh resolution probe and deuterochloroform as a solvent). In the caseof the prot on-NMR analysis, copolymer composition is calculated fromthe relative intensities of the chloromethyl proton signal. Molecularweight and molecular weight distribution were determined by gelpermeation chromatography () ) using a Waters 150-C instrument withtetrahydrofuran (Aldrich) as the carrier solvent. Weight averagemolecular weights (M_(w)) and polydispersities were found to range from98,000 to 130,000 and 2.5 to 3.2 respectively.

The S-VBC copolymer precursors of Example E are predominantly of therandom type as deduced from differential scanning calorimetry (DSC)measurements (only one transition was observed) as well as viaexamination of the reactivity ratios reported in the literature.

Evidence for the formation of the amino-copolymer is provided byelemental analysis and ¹³ C-NMR. For example, the spectra shown in FIGS.8A and 8B indicate that the alkylation reaction is quantitative. Theproton-NMR spectra reported in FIGS. 9A and 9B also support thisconclusion.

b. Effect of composition upon glass transition temperture

Incorporation of significant amounts of a comonomer into polystyrene canchange the glass transition temperature of the polymer, although thedegree of change depends on the side-group structure and concentration.Differential Scanning Calorimetry (DSC) measurements the glasstransition of the amino-functionalized copolymers of Examples A and B(derived from 1:5 VBC/styrene precursor) are set forth in Table 1 below.A range of Tg's are exhibited.

                  TABLE 1                                                         ______________________________________                                        Amine Group      Tg                                                           ______________________________________                                        PIP              123                                                          1,3-DAC          101                                                          EDA (Scheme I)   83                                                           1-MPIP           120                                                          1,2-DAC          85                                                           DETA             82                                                           Base copolymer,  103                                                          S - VBC = 5:1                                                                 ______________________________________                                    

Glass transition temperatures (T_(g)) of the copolymers of Example Ewere evaluated with a TA 2000 thermal analysis system, using a heatingrate of 10° C./min. T_(g) 's of the copolymers were determined fromsecond scans. The glass transition temperatures of the precursors werefound to vary with copolymer composition as shown in FIG. 10.

4. Reaction of Amino-Functionalized Polymers with CO₂

Amine functionalized styrene-based copolymers were found to reactreadily with carbon dioxide, either in bulk or solution using thefollowing methods:

1. Copolymer powder or a compression molded disk or film (13 mm indiameter) may be exposed to carbon dioxide at ambient temperature andpressure, at the vapor pressure, or at supercritical conditions.

2. The animated copolymer may be dissolved in chloroform under ambientconditions. Upon sparging with CO₂ a white precipitate quickly appears,indicating the formation of the polymer-bound zwitterion. The product isseparated by filtration and dried under vacuum.

The aminated copolymers of Table 2 below were complexed with CO₂ viamethod 1. The stability of the CO₂ adduct was analyzed usingthermogravimetric analysis (TGA). TGA measurements on the CO₂ reactionproducts of the aminated copolymers as set forth in Table 2 show thatthe onset of weight loss occurs at temperatures above 60° C. Theanalysis of the aminated copolymer prior to reaction with carbon dioxiderevealed no weight loss up to 275° C. Examination of the TGA samplesfollowing completion of the temperature program clearly showed a foamedmorphology.

                  TABLE 2                                                         ______________________________________                                                  Onset        Complete Weight                                        Amine     (°C.) (°C.)                                                                           Loss, %                                       ______________________________________                                        EDA       62           140      9.7                                           1,4-DAC   65           140      4.8                                           PIP       78           170      8.4                                           ______________________________________                                    

The amount of carbon dioxide fixed by the animated copolymers of ExampleE upon exposure to the gas was measured by TGA using a 0.100 ml platinumpan and operated at a heating rate of 10° C./min using a nitrogen gaspurge at 50 ml/minute through the balance and furnace compartments.

The chemical composition of the gases evolved upon heating wasdetermined by mass spectroscopy using a Dycor quadruple instrument.After exposing an amino-copolymer in the bulk state to CO₂ at its vaporpressure, the product of the reaction was sealed in a vial under anitrogen atmosphere. Mass spectroscopy data were collected by firstsampling the ambient air for 1 minute. Immediately thereafter, thesampling capillary was inserted into the vial and the gases inside thevial were analyzed at ambient temperature for 1 minute. Finally, thevial was immersed in an oil bath preheated to 90° C. and data taken foranother 15 minutes. In the course of this experiment, the instrument wasset to monitor carbon dioxide, water and nitrogen.

Fourier transform infrared spectroscopy (Mattson Polaris) was used toprobe for functional groups present at various stages of the CO₂fix-release cycle. Samples for the FTIR experiments were prepared in theform of thin films cast from a solution of the aminated copolymer inchloroform. FTIR spectra of virgin film and that exposed to liquid CO₂for several hours were recorded. Then the exposed sample was heated to80° C. for 5 minutes in a vacuum oven and its FTIR spectrum is recordedagain.

The glass transition temperatures of the amino-functional copolymerswere found to be a function of amino structure as shown in Table 3 andamine content as indicated in FIG. 11.

                                      TABLE 3                                     __________________________________________________________________________    Glass Transition Temperatures of Amine-Copolymers                             as a Function of Structure and Composition                                                               Comonomer Mole Fraction                            Amine Type                                                                             Structure         0.13   0.24                                        __________________________________________________________________________    EDA      H.sub.2 N(CH.sub.2).sub.2 NH.sub.2                                                              101    92                                          N-MEDA   HNCH.sub.3 (CH.sub.2).sub.2 NH.sub.2                                                             84    90                                          N,N-DMEDA                                                                               ##STR7##         103    91                                          N,N'-DMEDA                                                                             H.sub.3 CNH(CH.sub.2).sub.2 NHCH.sub.3                                                          101    79                                          N,N,N'-TMEDA                                                                            ##STR8##          97    94                                          TETA     H.sub.2 N(CH.sub.2).sub.2 NH(CH.sub.2).sub.2 NH(CH.sub.2).sub.2               NH.sub.2          --     70                                          __________________________________________________________________________

As can be seen in FIG. 11 low amounts of EDA-functional comohomer act toplasticize the polymer, lowering T_(g) from 102° C. (styrenehomopolymer) to 80° C. at 40 mole %, further addition of EDA-functionalcomonomer dramatically increases T_(g), such that a 50/50 copolymerexhibits a T_(g) of 103° C., and copolymers with high functionalcomonomer content (60, 75 and 100%) did not show any thermal transitionwithin the range of -60° to 300° C. These polymers are readily solublein chloroform/methanol mixtures; thus it is not likely that crosslinkinghas produced this anomalous T_(g) behavior. TGA measurements revealedthat these materials undergo no weight loss up to 300° C. indicating athermal stability comparable to that of a styrene homopolymer.

Preferably, therefore, the functional comohomer content is kept belowapproximately 40%. Most preferably the functional comonomer content isbetween approximately 20% and 40% (e.g. x:y in FIGS. 1, 2, 4, 5 and 6preferably is in the range of approximately 4:1 to 3:2).

Following exposure to carbon dioxide, the amino-functional polymers ofExample E were allowed to age at atmospheric conditions for 24 hoursprior to TGA experiments. Typical CO₂ desorption curves recorded by DSCand TGA are shown in FIGS. 12A and 12B. As can be seen, the weight lossresulting from CO₂ release occurred abruptly and was complete relativelyquickly. The non-linear dependency of adsorbed CO₂ weight fraction (seeFIG. 13) and amine content is believed to be a result of CO₂ diffusionlimitations, poorer solubility of the copolymer at higher amine loadingsand inaccessibility of amine sites entrapped in the bulk of theprecipitate during reaction with CO₂.

The amount of carbon dioxide bound by these copolymers is a function ofamine structure as shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        CO.sub.2 Binding as a Function of Amine Structure                             and Reaction Conditions                                                                         Bound CO.sub.2 Weight %                                     Amine Type        Method A  Method B                                          ______________________________________                                        EDA               3.8       13                                                N-MEDA            1.7       5.6                                               N,N-DMEDA         1.9       4.2                                               N,N'-DMEDA        1.0       4.0                                               N,N,N'-TMEDA      1.1       1.8                                               Polystyrene Homopolymer                                                                         <0.1                                                        ______________________________________                                         Method A: Aminocopolymer exposed to liquid CO.sub.2 at its vapor pressure     Method B: Aminocopolymer exposed CO.sub.2 at 1 atmosphere in chloroform       solution                                                                 

The data suggest the following generalizations:

1. Binding capacity of pendant amine groups increases as amine basicity(relative to a Lewis acid such as CO₂) increases; i.e., primary >secondary > tertiary.

2. Thermal stability of amine-CO₂ adducts appears to increase uponattachment of the amine to the polymer backbone, in that a tertiaryamine-functional polymer binds CO₂ (albeit a small amount) whereas a lowmolecular tertiary amine does not.

3. Placement of the amine groups (relative to the phenyl groups) haslittle or no effect on CO₂ -binding capacity (compare results forN,N-DMEDA (secondary-tertiary} and N,N'-DMEDA (tertiary-secondary}).

As shown by the TGA and DSC scans, the onset of catastrophic thermaldissociation occurs at 70° C. and is complete at 160° C. (at ambientpressure). Using a high pressure DSC cell it was observed that the CO₂desorption endotherm is shifted to higher temperatures when the testpressure is increased as shown in FIG. 14. Visual as well as microscopicexamination of the sample residues revealed that these products exhibita foamed structure.

Mass spectroscopy experiments, in parallel with FTIR spectroscopy,proved to be a useful combination for the characterization of thereaction products and also for the investigation of possibleside-reactions (such as the potential for urea formation). Typicalspectra (see FIG. 15) do not exhibit absorption bands characteristic ofurea functionalities (at 1650 cm⁻¹ in the case of diethylurea);suggesting that the decarboxylation step results in a clean regenerationof the original aminated copolymer. Further evidence in support of thisobservation is provided by the mass spectroscopy data presented in FIG.16. Upon heating the CO₂ /EDA-copolymer reaction products, the carbondioxide signal increased by several orders of magnitude, whereas thewater signal remained unchanged throughout the course of the experiment.Also, cyclic reactions of adsorption versus thermally induced desorptionof CO₂ are fully reproducible, suggesting that the process of reactingthe regenerating the amino sites proceeds cleanly.

Acid Gas Sotbents and Microporous Acid Gas Sorbents.

As shown above, linear copolymers incorporating pendant amino-groupsreadily fix CO₂ at ambient conditions, reverting to the originalaminated polymer by releasing the polymer-bound CO₂ when heated totemperatures above 100° C. in general. It has been discovered thatpolymers incorporating amino-groups are good sorbents for acid gasesother than CO₂. Linear and branched polymers for use as acid gassorbents preferably have a M_(w) greater than approximately 5000. Morepreferably, M_(w) is greater than 10,000.

It has also been discovered that aminated polymers used as sorbents foracid gases are preferably formed as microporous beads to increase boththe rate of binding and the binding capacity as a result of increasedsurface area. The polymer porosity is thus preferably maximized.

The surface area of linear aminated polymers can be advantageouslymaximized for use as acid gas sorbents by depositing such polymers as acoating or film upon a substrate. For example, the aminated polymer canbe deposited as a coating upon glass beads.

As shown in FIG. 17, such microporous polymers are preferably preparedby solution polymerization of divinylbenzene-vinylbenzylchloride(DVB-VBC) and divinylbenzene-styrene-vinylbenzylchloride (DVB-S-VBC)systems to produce crosslinked microporous beads. As is the case withthe linear precursors, the microporous copolymer precursors are thenanimated, producing polymeric sorbents with a variety of structures andtopographic morphologies. Under ambient conditions all accessible aminegroups are capable of reacting with acidic substrates to form relativelystable compounds, and complete desorption of the bound species can beaccomplished by heat-conditioning, preferably at approximately 110° C.

The effects of particle size, BET surface area and amine content on thekinetics of, and ultimate capacity for acid gas binding by linear andcrosslinked amino-polymers have been investigated. To explore thesignificance of mass transport limitations on the reaction kinetics, aporous-crosslinked beads and their equivalent non-porous linearcounterparts were comparatively evaluated.

1. Synthesis of DVB-VBC crosslinked porous beads.

VBC (70% meta, 30% para), styrene and divinylbenzene (DVB, Aldrich) wereeach washed several times with a 0.5% aqueous sodium hydroxide solutionto remove the polymerization inhibitors, and then rinsed with distilledwater until the washings tested neutral to litmus paper. Molecularsieves were added to the monomers, which were then stored at 4° C. Priorto polymerization, the monomers were purified by distillation undervacuum (30 in. Hg), at 40° C. for styrene and DVB and 90° C. for VBC.

The crosslinked precursor was prepared in a toluene (Fisher) solutionvia a free radical polymerization initiated by AIBN (Aldrich). In atypical recipe, a 1000 ml three-neck round-bottom flask mounted with areflux condenser was flushed with nitrogen for 2 hrs. The reactor wasthen charged with 50 ml toluene, 13 ml VBC (91.2 mmol), 7 ml DVB (49mmol) and a solution of 23 mg AIBN in 3 ml toluene. The reactor wasimmersed in an oil bath preheated to 100° C. and stirred vigorously for8 hours. The precursor colloidal dispersion was de-stabilized by pouringit into a large volume of methanol. The polymer was recovered byfiltration, washed several times with methanol, re-dispersed inchloroform and re-coagulated in methanol. The product was then dried ina vacuum oven at 100° C.

2. Preparation of amine functional porous beads.

The crosslinked porous precursors were functionalized with EDA bynucleophilic substitution in a toluene solution (40° C.) under nitrogenin the presence of a polymer supported polyDMAP. In a typical amination,35 ml EDA (525 mmol, Aldrich) and 40 ml toluene were placed in a 250 mlthree-neck round-bottom flask previously flushed with nitrogen. Thecolloidal suspension of the DVB-VBC precursor (50 ml) was then addeddropwise to the EDA solution while mixing. The slurry was stirred for 48hrs and then heated to 80° C. for 4 hrs. To recover the product, thecontents of the reactor were poured into a large volume of distilledwater, stirred vigorously, after which the solvent was evaporated undervacuum. The polymer, in the form of a white powder, was washed withdistilled water several times until the washings tested neutral tolitmus paper, and then dried in a vacuum oven at 100° C.

3. Product Characterization

The chemical composition of the crosslinked beads was determined byelemental analysis for nitrogen and chlorine at Galbraith Laboratories;Knoxville, Tenn. High resolution proton-NMR was used to complement theelemental analysis results. Fourtier transform infrared spectroscopy(FTIR) (Mattson Polaris instrument) was also used to probe the chemicalstructure of the DVB/VBC precursor and the amino-functional product.Spectra were recorded on solid samples prepared in the form of KBrwafers.

The chemical compositions of the polymer precursors and theamino-functional crosslinked beads were determined via elementalanalysis of nitrogen and chlorine. The results of these measurements,set forth in Table 15 below, indicate that as the mole fraction of thecrosslinker (DVB) increases, the conversion of chloromethyl toaminofunctional product decreases. This result arises from a lowerdegree of swelling of the polymer by the amine solution because ofhigher crosslink density.

                  TABLE 5                                                         ______________________________________                                        Sample:          BEAD 1   BEAD 2   BEAD 3                                     ______________________________________                                        DVB (ml in feed) 7        7.5      7                                          VBC (ml. in feed)                                                                              13       10       3                                          S (ml. in feed)  0        0        0                                          Nitrogen (weight %)(*)                                                                         7.84     59       3.62                                       Chlorine (weight %)(*)                                                                         .68      1.21     .79                                        Conversion (%)   94       86       85                                         Functional comonomer mole                                                                      .43      .32      .18                                        fraction                                                                      BET surface area (m.sup.2 /g)                                                                  49       43       45                                         ______________________________________                                         .sup.(*)Evaluated by elemental analysis at Galbraith Laboratories,            Knoxville, TN.                                                           

Consequently, at low VBC mole fractions, styrene spacers were insertedinto the network to maintain similar crosslink densities and thus ensuresimilar chemical structures. Further, attempts to prepare crosslinkedprecursors with a VBC to DVB mole ratio higher than that of theprecursor of BEAD 1 of Table 5 resulted in soluble, non-crosslinkedmaterials, i.e., branched copolymers of VBC and DVB.

Typical FTIR spectra of solid KBr/copolymer wafers as shown in FIG. 18indicate the formation of an amine-functional product as evidenced bythe appearance of a medium absorption band as a doublet indicative ofprimary amine groups and centered at 3400-3200 cm⁻¹ (N-H stretch) andthe characteristic C-N stretch band at 1120- 1110 cm⁻¹. Spectra recordedon the DVB/VBC crosslinked precursors also showed the presence of a C=Cstretch at 1630-1620 cm⁻¹ arising from unreacted vinyl groups of the DVBunits. Thermal analysis of crosslinked beads revealed no transitionwithin the temperature range -80° to 300° C. and no weight loss by TGAup to 300° C.

4. Binding studies of acid gases by aminated polymers

The ability of the aminated polymers to bind carbon dioxide reversiblywas tested by a number of means including mass spectroscopy (Dycorquadrupole apparatus). The binding step was carried out in a batch modea using 250 ml flask loaded with the aminated polymer (130 mg, 0.33 mmolamine) which was heated to 90° C. in an oil bath for 30 minutes under asteady flow of nitrogen purge gas (1000 ml/min) to ensure that allpolymerbound CO₂ (formed upon exposure to air) was evolved. Aftercooling the reactor, CO₂ (0.3 ml, 0.012 mmol) was injected into theflask. The ability of these aminated polymers to bind CO₂ was assessedby measuring the CO₂ concentration in the reaction flask. Debonding ofCO₂, and thus regeneration of free amine sites, was confirmed bymonitoring the CO₂ signal upon heating of the polymer to 75° C. For thecases of SO₂ and NO, thermogravimetric analysis (described below) wasused to assess the ability of the polymeric sorbents to bind these gasescleanly and reversibly. The present aminated polymers have also beenshown for reversibly bind H₂ S.

5. Thermal analysis.

Reactions of acid gases (CO₂, SO₂, NO) with polymer-anchored amines werefollowed in a TGA apparatus (TA 2000 system) by exposing the polymerpowder to the pure substrates as purge gases. Gas mixtures withconcentrations reflecting those encountered in typical stack emissions(14% CO₂, 1% SO₂ and 0.5% NO, each balanced with nitrogen) were used.Because the aminated polymers fix CO₂ from the atmosphere, polymers werethermally regenerated prior to all binding studies. The full procedurefor examination of acid gas binding experiments is given below:

1. Set the nitrogen purge rate at 200 cm³ /min.

2. Ramp at 10° C./min. from room temperature to 105° C., or to atemperature 5° C. below Tg (glass transition) for the linear polymers.

3. Maintain isothermally for 30 minutes.

4. Equilibrate at 80° C.; maintain under isothermal conditions for 1200minutes.

5. Equilibrate at 25° C., then reduce the nitrogen purge rate to 50 cm³/min and maintain isothermally for the remainder of the experiment.

6. Switch purge gas to CO₂, SO₂, H₂ S, NO₂ or NO. Complete reaction ofall accessible amine sites with the acid gas was considered to have beenattained when no weight change within the sensitivity limits of theinstrument is detected within a 60 minute time interval.

The heat of reaction of the acid gas binding process was measured by DSCusing a standard cell, CO₂ as purge gas at a rate of 50 cm³ /min, andpolymeric sorbent samples in the bulk state contained in open pans. Inthe experiments the most critical step is the precise measurement of thesample weight in that the sample to be analyzed must be free of anyabsorbed substrates when measuring the weight. As a result the TGAinstrument was used to determine the sample weight following theprocedure below:

1. Set up the instrument under isothermal conditions at 25° C. and anitrogen purge rate of 50 cm³ /min.

2. Place an empty DSC pan in the TGA and zero the instrument.

3. Place the polymeric sorbent sample inside the DSC pan and reload thewhole into the TGA instrument.

4. Heat-treat the sample as described previously from step 1 to 5.

5. Record the sample weight during the course of step 5. The DSC pan isthen removed and loaded carefully into the DSC cell. The sample is againheat-treated using the same procedure to drive off any bound-CO₂ duringthe brief exposure of the sample to ambient air prior to switching toacid gas purge.

In the study of CO₂ debonding by DSC, the polymer was exposed to CO₂ atits vapor pressure in a high pressure reactor for two minutes and thenallowed to age at ambient conditions for several hours. The product wasthen encapsulated in an aluminum pan and its thermogram recorded using a50 ml/min nitrogen purge and a heating rate of 10° C./min.

Thermal analysis of the uncrosslinked materials showed no weight lossbelow 300° C., but revealed curious behavior regarding the effect ofamine content on the glass transition temperature (T_(g)). As shown inFIG. 11 as amine content initially increases, T_(g) decreases due to thefamiliar side-chain plasticization effect. However, at an amine contentof approximately 40% , T_(g) exhibits a minimum, and quickly rises to apoint above 300° C. as amine content approaches 60%. The high aminecontent copolymers are not crosslinked, as they were recovered fromhomogeneous solutions after their synthesis. Consequently, it issurmised that strong interchain interactions between amines and phenylgroups are behind this anomalous T_(g) -behavior.

The porosity of the polymeric sorbents was determined by measuring theirsurface area per gram of product using the BET sorption technique. Thelinear copolymers were found to have surface areas ranging from 28 to 33m² /g, consistent with values characteristic of non-porous materials.The crosslinked beads were found to have higher surface areas than theirlinear counterparts (see Table 5) and clearly exhibit a porousmorphology as shown by electron micrographs.

6. Cyclic binding-debonding of acid gas.

Porous crosslinked polymer beads carrying pendant amino-groups bindcarbon dioxide when exposed to ambient air and undergo reversiblefix-release cycling with CO₂ without alteration to their chemicalnature. Mass spectroscopy was used to follow the binding/debonding ofCO₂ by porous, crosslinked amino-functional polymers. As illustrated inFIG. 19, the spectrometer was first calibrated with gas mixtures of CO₂in nitrogen of known composition and then set to monitor CO₂. Theexperiments were conducted under ambient conditions after heat-treatingthe polymer as summarized previously. Ambient air was analyzed for 0.5minutes, then the sampling needle was inserted into the flask containing130 mg polymer sorbent (0.33 mmol amine) in an atmosphere of 250 mlnitrogen and 0.3 ml CO₂ (0.012 mmol).

As shown in FIG. 20, in the presence of the sorbent, the CO₂concentration was reduced to within the lower limits of detection of theinstrument within 2 minutes, revealing rapid CO₂ -binding by thesemicroporous materials. At time t=4 min, the flask was immersed in an oilbath at 75° C. and the CO₂ concentration increases rapidly as CO₂ isreleased by the sorbent via thermal dissociation (thus regenerating thefree amine groups of the polymer). At time t=8 min, the flask wasremoved from the oil bath and immersed in a water bath at roomtemperature. Upon cooling of the system, CO₂ bound again to the polymer,as evidenced by the quick decrease of CO₂ concentration in the flask.

In the case of SO₂ and NO, the cyclic binding/debonding processes wererecorded with TGA. Typical results, as shown in FIGS. 21A-21D, provideconclusive evidence for the ability of the sorbents to fix thesesubstrates cleanly and reversibly. Furthermore, the total number ofmoles of bound SO₂ was more than double that of CO₂, presumably owing tothe higher acidity of the former substrate. Similar observations arealso applicable to nitric oxide as seen in FIGS. 21C and 21D. Using a0.5% NO mixture with a nitrogen balance, a 6% (and still increasing)weight increase due to NO binding were recorded after 4 days ofexposure. Moreover, FTIR spectra recorded on samples after severalcyclic exposures to acid gas showed no alteration in their chemicalstructure.

Carbon dioxide binding and debonding were also followed by DSC. FIG. 22shows a typical binding exotherm. The calculation of the heat ofreaction based on the integration of the exotherms for several samples(linear copolymers with 20%, 40%, 75% functional comonomer molefractions and the crosslinked beads with 40% functional comonomer molefraction) gave an average value of 14 kcal/mole with an error of ±7%.Similarly, evolution of CO₂ was characterized by a large endotherm (seeFIG. 23) whereas a second heat-up of the same sample does not show anythermal effect. A salient characteristic of these polymers is that uponexposure to ambient air even for a brief period of time, a large CO₂-degassing endotherm is always observed during the first heat-up in theDSC cell; a further indication of the high affinity of these materialsto bind CO₂ from the air.

The CO₂ -binding capacity of these polymers was evaluated at varioustemperatures ranging from 25° to 105° C. Since the reaction of CO₂ withthe polymer amino-groups is exothermic one would expect the CO₂ -bindingcapacity of these polymers to be highest at low temperatures (see FIG.24). FIG. 24 also indicates that CO₂ does not bind to the polymer attemperatures exceeding 105° C. (under atmospheric pressure). It isimportant to note that a true equilibrium constant for these reactionscould not be calculated because the reactive sites in the polymerparticles are not equally accessible for reaction. FIG. 25 shows that atlow amine content (below 40% functional comohomer) the mole fraction ofreacted nitrogens is proportional to the functional comonomer molefraction. Above 40%, the curve levels off, indicating that thefunctional comohomer mole fraction has no more effect on the molefraction of reacted amine groups.

7. Effect of mass transport on CO₂ -binding.

CO₂ binding in EDA-functional polystyrene is primarily due to theprimary amine group, which fixes approximately 0.30 CO₂ 's per nitrogen,while the secondary nitrogen only binds 0.06. Further, the linearcopolymers bind more CO₂ when reacted in solution than in the bulkstate, leading to the conclusion that the binding capacity of thesepolymeric sorbents is governed by the concentration of accessible aminegroups. As shown in FIG. 26 this accessibility criterion for a highbinding capacity becomes more striking when comparing the porous andnon-porous sorbents with the same functional comonomer mole fraction.Upon exposing these products to gaseous CO₂ in the TGA, the CO₂ bindingcapacity for the porous beads is more than double that of the linear nonporous copolymers, owing to the large BET surface area of the porousbeads and thus a higher surface concentration of accessible bindingsites. In fact, the TGA data showed that 41% (molar) of the totalnitrogen content for the porous materials have reacted with CO₂,corresponding to an 80% conversion based on terminal nitrogens.Conversely, only 20% of the nitrogens of the linear non-porous copolymerparticipated in the binding reaction corresponding to a 40% conversionbased on the same considerations. These observations suggest that themajority of the binding sites of the linear non-porous copolymers areburied in the polymer matrix.

The shape of the binding capacity curves in FIG. 26 for the linearcopolymers reacted with CO₂ in the TGA show that the binding capacity ofthese sorbents levels off at high functional comonomer fraction. Thisbehavior may be attributable to variations in polymer microstructure asamine content increases, and consequently the CO₂ transmittance (C*D,where C is concentration and D is the diffusion coefficient). Assumingthat the CO₂ diffuses into the polymer spheres uniformly, forming aspherical shell of CO₂ :polymer product, one can derive the thickness ofthe shell formed from such quantities as the fractional weight increase(FWI) from the TGA data (at 25° C.) and the nitrogen weight fraction (N)of the polymer from elemental analysis. Assuming that the amine groupsare distributed evenly through the polymer, and that 1 CO₂ molecule isbound by each accessible tail amine group, equation 6 is derived: (SeeAppendix for derivation) ##EQU1## where d is the thickness atequilibrium of the shell that has reacted with CO₂ and R_(p) is theaverage radius of the non-porous polymer particles. A plot of d/R_(p) vsfunctional comonomer mole fraction provided in FIG. 27 highlights theonset of a facilitated transport mechanism of CO₂ as amine content inthe polymer is increased. Maximum facilitated diffusion occurs at afunctional comonomer mole fraction of 0.4. It is important to note thatat about 40% functional comonomer content, the curve of d/R_(p) vs Nchanges curvature, suggesting the onset of diffusional constraints, theposition of which Corresponds closely to the onset of the dramaticincrease in the Tg of the aminated copolymers mentioned previously. Alarge increase in the Tg of the polymer, while maintaining a constantoperating temperature, would be accompanied by a drop in chain-segmentmobility, and thus a decrease in the diffusion coefficient.

Observation of an apparent facilitated transport mechanism promptedexamination of the kinetics of CO₂ reaction in more depth. The existenceof three kinetic regimes, as depicted in FIGS. 28A and 28B, waspostulated for a typical binding reaction monitored by TGA. First,diffusion of CO₂ from the bulk to the polymer surface, followed by fastreaction (regime I). Second, after sufficient time has elapsed to reactall of the surface sites, further binding of CO₂ will proceed viadiffusion through a thin film of reacted polymer to unreacted sites(regime II). Third, upon formation of a sufficiently thick shell,further binding will be governed by diffusion through a substantialspherical layer of amino-polymer:CO₂ product (regime III), followed byfast reaction.

In regime III, it is postulated that the rate of CO₂ binding isdetermined by the rate of CO₂ diffusion through the product layer. Toobtain an estimate of the diffusion coefficient of CO₂ through theproduct layer in regime III, the TGA data collected at very long timeswithin the diffusion-controlled reaction regime were plotted using amodel derived by Souchay and Pannetier, Chemical Kinetics, Elsevier,(1967), the disclosure of which is incorporated herein by reference:

    (FWI)-1.5*(FWI).sup.2/3 =(Cons.) *t                        (7)

which yields a straight line (see FIG. 29), the slope of which isproportional to the diffusion coefficient (see FIG. 30). At low amineloading, CO₂ intra-particle diffusion is enhanced by facilitatedtransport, whereas at higher amine contents the CO₂ :polymer complexcoat surrounding the unreacted core of the sorbent particles imparts agradual increase in the CO₂ barrier properties of the reacted shell asfunctional comohomer content is increased from ca. 40 to 100%. It can beclearly seen that optimum facilitation occurs, as stated earlier, in therange of 0.4 functional comonomer mole fraction, coinciding with theminimum of the Tg-amine relationship for these materials. Higher amineloadings cause the diffusion coefficient to fall sharply until it isapproximately equal to that of glassy polystyrene.

8. Reaction kinetics of acid gas binding.

In regime I, the reaction of acid gases such as CO₂, SO₂, H₂ S or NO_(x)with the polymeric sorbents under isothermal conditions, as describedearlier, lends itself to a simple mathematical treatment using apseudo-first order kinetic model. Since the binding reactions arecarried out under a steady flow of the gas being studied, it islegitimate to assume that the bulk concentration of the acid gas in theTGA furnace remains constant at all times. In this model, the totalnumber of free amine sites [S_(o) ] capable of participating in acid gasbinding is given by the number of primary amine groups present in thepolymer which is given by: ##EQU2## where m_(p) is the mass of theunreacted polymer. At any arbitrary time of the reaction, the number ofvacant sites [S] is estimated from the absorption isotherm as follows:##EQU3## Under a constant acid gas concentration, integration of thefirst order reaction model leads to: ##EQU4## where k is a lumped rateconstant. Substituting the appropriate masses of bound CO₂, SO₂ or NOfor [S] and [S_(o) ] yields the more convenient expression: ##EQU5##where m_(c) is the mass of bound substrate at time t. At the earlystages of the reaction, FIG. 31 suggests that the binding processfollows pseudo-first order kinetics. At higher amine contents however,diffusional limitations become rate-limiting and thus the gradualslow-down in the reaction rate results in a deviation from linearity.Similar conclusions are also applicable to the porous materials althoughin this instance pore and intra-particle diffusion restrictions are therate determining factor at longer times.

The kinetic analysis of regime I would be incomplete without a study ofthe mass transport effects through the stagnant gas film surrounding thesorbent particle. If the reaction rate were governed by the diffusionrate of CO₂ through the gas film, the progress of the reaction wouldfollow the expression: ##EQU6## where f is the fraction conversion, t istime and: ##EQU7## is the time required for complete reaction and k_(g),is the mass transfer coefficient of CO₂ through the gas film. Hence, aplot of the fraction conversion as a function of time should yield astraight line whose slope is proportional to k_(g) (FIG. 32). It isexpected that this model would also fit the experimental data becausethe mathematical expressions describing the kinetically controlled andthe mass transport controlled models are identical in form at lowconversion. In order to unequivocally find out whether the bindingreactions in regime I are mass transfer or kinetically controlled, theconcentration of CO₂ in the gas purge was varied in order to determineits effect on the rate of CO₂ binding. If regime I were mass transportlimited, the rate of CO₂ binding would be proportional to the CO₂concentration in the purge, that is:

    Rate=(Const.)*k.sub.g *[CO.sub.2 ]                         (14)

assuming that k_(g) is not a function of CO₂ concentration. A plot ofthe rate of CO₂ binding versus CO₂ concentration in FIG. 33 does notyield a straight line which indicates that the rate of CO₂ binding inregime I is not limited by mass transport of CO₂ through the gasstagnant film. It is therefore concluded that regime I is kineticallycontrolled.

Crosslinked amino-functional polymers in the form of porous beads (40 m²/g) were found to exhibit affinity and selectivity for the adsorption ofacidic gaseous substrates. Their binding capacity was found to besuperior to their non-porous (below 30 m² /g) linear counterparts owingto a much larger surface area and thus a greater concentration ofaccessible surface binding sites per gram of polymer. The reactionkinetics were found to follow a pseudo-first order process at theinitial stages of the reactions and become mass transport limited atlonger times. The dramatic changes in the CO₂ transmittance (C*D)through the reacted polymer coat surrounding the sorbent particlessuggest the onset of a CO₂ -transport facilitation mechanism at lowamine loading. As amine content is increased above the range of 40%comonomer mole fraction, the diffusion coefficient of CO₂ through theproduct layer decreases sharply indicating the formation of a materialwith high CO₂ -barrier properties.

CO₂ binds readily onto the amine sites when exposing these materials toambient air. Reversibility of the absorption-desorption processes forCO₂, SO₂, H₂ S and NO occur in a clean and complete fashion under mildconditions thus making this class of materials potentialenvironmentally-friendly sorbents for acid gas pollutants.

9. Acid gas removal

As shown by the above studies, acid gases including CO₂, NO_(x), SO₂ andH₂ S can be removed by linear aminated polymers, branched aminatedpolymers and preferably crosslinked, microporous aminated polymerssimply by contacting a gas stream or system containing one or more acidgases to be removed with such aminated polymers in solid form. The acidgases are preferably complexed or reacted with the aminated polymers attemperatures well below the temperature at which the products of thereaction of aminated polymers and acid gases thermally dissociate ordebond to produce the free acid gas and a regenerated aminated polymer.

Preferably, the aminated polymer is in the form of microporous beads.Operational units for effecting removal of at least a portion of an acidgas from a gas stream containing the acid gas are illustrated in FIGS.34A and 34B. Generally, operation units 10 and 20 comprise a vessel 100.Vessel 100 comprises an inlet means 110 for receiving the gas stream andan outlet means 120 for releasing the gas stream after removal of theportion of the acid gas therefrom. Operational units 10 and 20 alsocomprise a sorbent system comprising an aminated polymer 200 containedwithin vessel 100 in a manner that the gas stream contacts the sorbentsystem to remove the acid gas from the gas stream by reactive complexingof the acid gas by the amine groups of the aminated polymer.

As shown in FIG. 34A the sorbent system can be contained in vessel 100as a fixed bed. Preferably, the sorbent system is fluidized as shown inFIG. 34B.

Preferably, the present method of acid gas removal is practiced in anoperational unit including a fixed bed or a fluidized bed of aminatedpolymer. As shown in FIG. 35, an aminated-polymer coated substrate 300can also be used. Preferably the substrate is in the form of a sphere310 having an aminated polymer film 320 on the exterior thereof.Preferably the spherical substrate 310 is a glass bead.

Although the invention has been described in detail for purposes ofillustration, it is to be understood that such detail is solely for thatpurpose and that variations can be made therein by those skilled in theart without departing from the spirit and scope of the invention exceptas it may be limited by the claims.

Appendix

The fractional weight increase (FWI) due to CO₂ reaction given by:##EQU8## where: m_(c) increase, and m_(p) =weight of unexposed polymer.The weight of the unexposed particle is given by: ##EQU9## where densityrefers to the density of the unexposed polymer. The weight increase ofthe particle due to CO₂ binding by tail amino-groups is: ##EQU10## Thefractional weight increase due to reaction with CO₂ is: ##EQU11##Rearranging this expression yields: ##EQU12##

What is claimed is:
 1. A method or removing an acid gas from a gassystem, comprising the step of: (a) exposing an acid gas sorbentcomprising an aminated polymer to the gas system under conditionssuitable to reactively complex the acid gas with said aminated polymer,said aminated polymer having a molecular weight greater than or equal toapproximately 5000, said aminated polymer comprising covalently attachedamino functional groups suitable to complex the acid gas.
 2. The methodof removing an acid gas from a gas system of claim 1 further comprisingthe step of:(b) heating said complexed aminated polymer to a temperaturesufficiently high to debond the acid gas.
 3. The method of removing anacid gas of claim 2 wherein steps (a) and (b) are repeated.
 4. Themethod of removing an acid gas of claim 1 wherein said at least one acidgas is complexed to pendant amine groups of said aminated polymer. 5.The method of removing an acid gas of claim 4 wherein said pendant aminegroups are selected from the group consisting of diamines and triamines.6. The method of removing an acid gas of claim 4 wherein said pendantamine groups are selected from the group consisting of primary andsecondary amines.
 7. The method of removing an acid gas of claim 5wherein said amines groups are selected from the group consisting ofprimary and secondary amines.
 8. The method of removing an acid gas ofclaim 1 wherein said aminated polymer comprises a copolymer of styreneand an amine functional vinylbenzyl moiety.
 9. The method of removing anacid gas of claim 1 wherein said aminated polymer comprises amicroporous, crosslinked polymer.
 10. The method of removing an acid gasof claim 9 wherein said microporous, crosslinked polymer comprises acopolymer of divinylbenzene and an aminated vinylbenzyl moiety.
 11. Themethod of removing an acid gas of claim 10 wherein said microporous,crosslinked polymer comprises a copolymer of divinylbenzene, styrene andan aminated vinylbenzyl moiety.